1. Field of the Invention
The present invention relates to a DC (direct current) motor drive circuit for use in a windshield wiper drive section or a power window drive section of automobiles, for example.
2. Description of the Prior Art
Heretofore, DC motor drive circuits using an electromagnetic relay have often been used in order to activate and control a windshield wiper drive section and a drive section for driving a power window mechanism to move a power window of automobile upward or downward
FIG. 1 of the accompanying drawings is a schematic circuit diagram showing an example of a prior-art DC motor drive circuit for use in a windshield wiper drive section. FIG. 2 is a schematic circuit diagram showing an example of a prior-art DC motor drive circuit for use in a drive section of a power window drive mechanism to move a power window upward or downward.
First, an example of a DC motor drive circuit for use in a windshield wiper drive section will be described with reference to FIG. 1. As shown in FIG. 1, one end of a windshield wiper driving DC motor 1 is connected to a terminal 2a connected to a movable contact (this movable contact is usually connected to a suitable means such as a contact spring driven by an armature) AR of an electromagnetic relay 2. The above terminal 2a connected to the movable contact AR will hereinafter be referred to as xe2x80x9cmovable contact terminalxe2x80x9d.
The other end of the DC motor 1 is connected to a terminal 2b connected to a normally closed contact N/C (i.e. break contact) of the electromagnetic relay 2. The above terminal 2b connected to the normally closed contact N/C will hereinafter be referred to as xe2x80x9cnormally closed contact terminalxe2x80x9d. A connection point 2d between the other end of the DC motor 1 and the normally closed contact 2b is connected to the ground.
A terminal 2m connected to a normally open contact N/O (i.e. make contact) of the electromagnetic relay 2 is connected to a power supply at a terminal 3, at which a positive DC voltage (+B) is connected from a car battery (not shown). The above terminal 2m to which the normally open contact N/O is connected will hereinafter be referred to as xe2x80x9cnormally open contact terminalxe2x80x9d.
The electromagnetic relay 2 includes a coil 2C to which a controlling current responsive to user""s operation is supplied from a windshield wiper drive controller 4 when the user operates a windshield wiper switch 5. The windshield wiper switch 5 includes three switching positions of xe2x80x9cOFF positionxe2x80x9d, xe2x80x9cINTERMITTENT positionxe2x80x9d and xe2x80x9cCONTINUOUS positionxe2x80x9d. Fixed contacts 5a, 5b, 5c at these switching positions are connected to the windshield wiper drive controller 4.
When the windshield wiper switch 5 connects its movable contact 5m to the fixed contact 5a (OFF position), the coil 2C is not energized by the controlling current from the windshield wiper drive controller 4 so that the electromagnetic relay 2 connects the movable contact AR to the normally closed contact N/C. As a result, one end and the other end of the DC motor 1 are connected to each other and thereby the DC motor 1 can be braked (or placed in the stationary state).
When the windshield wiper switch 5 connects the movable contact 5m to the fixed contact 5b (INTERMITTENT position), the coil 2C of the electromagnetic relay 2 is intermittently energized by the controlling current from the windshield wiper drive controller 4. As a result, the electromagnetic relay 2 connects the movable contact AR to the normally open contact N/O during the coil 2C is being energized by the controlling current. When the coil 2C is not energized by the controlling current, the electromagnetic relay 2 connects the movable contact AR to the normally closed contact N/C side. Specifically, the electromagnetic relay 2 alternately connects the movable contact AR to the normally closed contact N/C and the normally open contact N/O each time the coil 2C is energized or is not energized by the controlling current.
When the electromagnetic relay 2 connects the movable contact AR to the normally open contact N/O, direct current flows through the DC motor 1 as shown by a solid-line arrow I in FIG. 1 and thereby the DC motor 1 can be driven. When the electromagnetic relay 2 connects the movable contact AR to the normally closed contact N/C, the DC motor 1 can be braked. In other words, the DC motor 1 may be driven intermittently. As this DC motor 1 is driven intermittently, the windshield wiper is driven intermittently.
When the windshield wiper switch 5 connects the movable contact 5m to the fixed contact 5c (CONTINUOUS position), the windshield wiper drive controller 4 continuously supplies a controlling current to the coil 2C of the electromagnetic relay 2. As a result, the electromagnetic relay 2 connects the movable contact AR to the normally open contact N/O to permit the DC current to flow through the DC motor 1 continuously as shown by the solid-line arrow I in FIG. 1. Thus, the windshield wiper can be driven continuously.
When the windshield wiper switch 5 connects the movable contact 5m to the fixed contact 5a (OFF position), the coil 2C of the electromagnetic relay 2 is not energized so that the electromagnetic relay 2 is released to connect the movable contact AR to the normally closed contact N/C.
Next, an example of a conventional DC motor drive circuit for use in a power window drive section will be described with reference to FIG. 2.
As shown in FIG. 2, one end of a power window DC motor 11 is connected to a movable contact terminal 12a of an electromagnetic relay 12 that is used to move a power window upward. The other end of the DC motor 11 is connected to a movable contact terminal 13a of an electromagnetic relay 13 that is used to move a power window downward.
A normally closed contact terminal 12b of the electromagnetic relay 12 and a normally closed contact terminal 13b of the electromagnetic relay 13 are connected to each other. A connection point 12d between the normally closed contact terminal 12b and the normally closed contact terminal 13b is connected to the ground. A normally open contact terminal 12m of the electromagnetic relay 12 and a normally open contact terminal 13m of the electromagnetic relay 13 are connected to each other. A connection point 12e between the normally open contact terminal 12m and the normally open contact terminal 13m is connected to the power supply at the terminal 3, at which a positive DC voltage (+B) is connected from a car battery (not shown), for example.
A power window ascending controller 14 supplies controlling current to the coil 12C of the electromagnetic relay 12 each time the user operates a power window drive section to move the power window upward. A power window descending controller 16 supplies controlling current to the coil 13C of the electromagnetic relay 13 each time the user operates the power window drive section to move the power window downward.
While the user is operating the power window drive section to move the power window upward, a power window switch 15 is being energized and the power window ascending controller 14 supplies controlling current to the coil 12C of the electromagnetic relay 12 to energize the coil 12c to allow the electromagnetic relay 12 connect the movable contact AR to the normally closed contact N/O. Accordingly, direct current flows through the DC motor 11 in the direction shown by a solid-line arrow in FIG. 2 so that the DC motor 11 is driven in the positive direction, for example, to move the power window upward, i.e. in the direction in which the power window closes.
When the user stops operating the power window drive section to move the power window upward, a power window switch 15 is de-energized to stop the supply of the controlling current to the coil 12C of the electromagnetic relay 12 to allow the electromagnetic relay 12 to connect the movable contact AR to the normally closed contact N/C. Therefore, the DC motor 11 is braked to stop the upward movement of the power window.
While the user is operating the power window drive section to move the power window downward, a power window switch 17 is being energized and the power window descending controller 16 supplies the controlling current to the coil 13C of the electromagnetic relay 13 to energize the coil 13C to allow the electromagnetic relay 13 to connect the movable contact AR to the normally open contact N/O. Accordingly, direct current flows through the DC motor 11 in the direction shown by a dashed-line arrow 12 in FIG. 2 so that the DC motor 11 is driven in the direction opposite to the direction in which it is driven when the power window is moved upward thereby to move the power window downward.
When the user stops operating the power window drive section to move the power window downward, the switch 17 is de-energized so that the coil 13C of the electromagnetic relay 13 is not energized by the controlling current, permitting the electromagnetic relay 13 to connect the movable contact AR to the normally closed contact N/C side. Thus, the DC motor 11 can be braked and thereby the downward movement of the power window can be stopped.
In this manner, the conventional DC motor drive circuit uses one contact group of the electromagnetic relay and energizes the coil of the electromagnetic relay to connect the movable contact AR to the normally open contact N/O thereby to drive the DC motor. On the other hand, the conventional DC motor drive circuit de-energizes the coil of the electromagnetic relay to connect the movable contact AR to the normally closed contact N/C thereby to brake the DC motor.
In the electromagnetic relay for use in this kind of DC motor drive circuit, in the state in which the DC motor is driven by the direct current through the normally open contact N/O of the electromagnetic relay, if the coil is not energized by the controlling current so that the electromagnetic relay is released, then when the movable contact AR separates from the normally open contact N/O, an arc occurs between the normally open contact N/O and the movable contact AR. If the gap length between the movable contact AR and the normally open contact in the released state of the electromagnetic relay (hereinafter this gap length will be referred to as a xe2x80x9ccontact gap lengthxe2x80x9d for simplicity) is short, then when the electromagnetic relay is released, the movable contact AR is brought in contact with the normally closed contact N/C before the arc occurred as the movable contact AR is separated from the normally open contact N/O is cut off. As a consequence, the normally closed contact N/C and the normally open contact N/O of the contact group are short-circuited (shorted). There is then the risk that the electromagnetic relay will be degraded.
Accordingly, the contact gap length has been heretofore determined in accordance with the voltage (battery voltage) applied to the power supply at the terminal 3. Ordinary automobiles can be activated by a standard car battery of DC 12V and are able to drive the above-mentioned DC motor drive circuit by an electromagnetic relay in which the contact gap length is 0.3 mm, for example. On the other hand, large automobiles such as a truck and a bus can be activated by a car battery of a high voltage greater than 24V (maximum value is 32), for example. Therefore, such large automobiles require an electromagnetic relay in which the contact gap length is longer than 1.2 mm, for example, to drive the above-mentioned DC motor drive circuit.
Therefore, according to the conventional electromagnetic relay, since the contact gap length increases as the power supply voltage increases, it is unavoidable that the electromagnetic relay becomes large in size. Such large electromagnetic relay becomes troublesome when it is mounted on the printed circuit board. Moreover, since the stroke of the movable contact AR of such large electromagnetic relay lengthens, it is unavoidable that an operating speed of an electromagnetic relay decreases. In particular, recently, as so-called hybrid cars, which can be driven by an engine using electricity together with gasoline and electric cars become commercially available on the market, the voltage of the car battery becomes high increasingly. Therefore, the above-mentioned problem becomes considerably serious.
In view of the aforesaid aspects, it is an object of the present invention to provide a DC motor drive circuit in which the defect of the short caused by the arc can be avoided without increasing the contact gap length of the electromagnetic relay even when the voltage at the power supply increases.
According to an aspect of the present invention, there is provided a direct current motor drive circuit which is comprised of a contact group operated under control of an electromagnet created when a coil is energized, a direct current motor whose one end is connected to one end of a direct current power supply and a normally closed contact of the contact group and whose other end is connected to a movable contact of the contact group and one to a plurality of normally open contacts connected between one normally open contact of the contact group and the other end of the direct current power supply and openable and closable in unison with the one normally open contact.
In the DC motor drive circuit according to the present invention, when the controlling current is supplied to the coil of the electromagnetic relay in order to drive the DC motor and the movable contact is connected to normally open contact to permit the direct current to flow through the DC motor, the direct current is supplied through a plurality of normally open contacts connected in series to the DC motor.
Therefore, the circuit voltage obtained when the electromagnetic relay is released after the supply of the controlling current to the coil of the electromagnetic relay has been stopped, is applied to a plurality of gaps between the movable contacts (the movable contact is connected to the normally closed contact when the electromagnetic relay is fully released) and the normally open contacts connected in series. As a result, the voltage applied to each of the gaps is divided by the number of the normally open contacts connected in series and thereby the above voltage is decreased.
Therefore, when the supply of the controlling current to the coil of the electromagnetic relay is stopped and the electromagnetic relay is released, even if the arc occurs between the movable contact and the normally open contact N/O, the voltage applied to each of a plurality of gaps between the movable contacts and the normally open contacts connected in series decreases. Thus, even when the contact gap length is reduced, it is possible to avoid the problem of the short caused by the arc. In addition, since a plurality of movable contacts separate from a plurality of normally open contacts connected in series at the same time, the separating speed of the movable contact can increase equivalently.
As described above, according to the present invention, even when the small electromagnetic relay with the short contact gap length is used, the arc occurred when the electromagnetic relay separates the movable contact from the normally open contact can be cut off before the movable contact is returned to the normally open contact.
According to the present invention, it is possible to provide a DC motor drive circuit in which the arc cut-off capability can be improved much more by using a small electromagnetic relay whose arc cut-off capability is not sufficient.
In this specification, a capability for cutting off the arc occurred when the electromagnetic relay separates the movable contact from the normally open contact before the movable contact is returned to the normally open contact will be referred to as an xe2x80x9carc cut-off capabilityxe2x80x9d.